1. Field of the Invention
The present invention relates to a thin film transistor (TFT), and more particularly, to a thin film transistor having a high temperature substrate, and an organic EL display of an active driving type of the same, and a method for fabricating the same.
2. Background of the Related Art
In general, a semiconductor thin film used in the thin film transistor is formed by crystallizing an amorphous silicon film formed by plasma CVD or thermal CVD in a furnace, like an electric furnace, for more than 12 hours at a temperature higher than 600° C.
For obtaining a high quality semiconductor thin film having an excellent field effect mobility and a high reliability, it is required that the amorphous silicon film is heat treated for a long time.
Moreover, since it is required that a substrate in a display permits a light to transmit therethrough, the substrate is transparent. For fabricating the thin film transistor on the transparent substrate, it is required that a silicon semiconductor film is formed on the transparent substrate. Since glass, which can be used in a low temperature, is used as the substrate, it is required that the silicon semiconductor film formed on the substrate is amorphous, which can be formed at a low temperature. Then, the amorphous silicon is crystallized for obtaining a fast response of the TFT. However, the substrate can be deformed during a high temperature heat treatment of crystallization of the amorphous silicon film.
In general, the substrate of the TFT is formed of silica glass, or non-alkali borosilicate glass. The substrate of silica glass has an excellent heat resistance, to suit to a high temperature process. The silica glass shows no deformation in a heat treatment at approx. 1000° C. However, the silica glass is very expensive in comparison to non-alkali borosilicate glass, which is increased by a geometric progression as a size of the substrate is increased. Therefore, the silica glass is not suitable for a large sized display due to the high cost.
In the meantime, though the non-alkali borosilicate glass is not expensive compared to the silica glass, the non-alkali borosilicate glass is not suitable for a high temperature process due to a poor heat resistance. That is, the non-alkali borosilicate glass deforms even in a heat treatment at approx. 600° C., which is particularly significant for a large sized substrate with a diagonal length greater than 10 inches.
Accordingly, though the non-alkali borosilicate glass has mostly been used for displays owing to the low cost, performance of the thin film transistor is not good due to low temperature crystallization of the semiconductor.